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- Charting New Pathways to Form Wolf–Rayet Stars at Low Metallicities
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Charting New Pathways to Form Wolf–Rayet Stars at Low Metallicities
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Low-metallicity Wolf–Rayet (WR) populations, such as the one of the Small Magellanic Cloud (SMC), are expected to be slightly influenced by metallicity-dependent effects, such as envelope inflation, which typically positions stars at cooler temperatures. Consequently, these populations should be easier to understand from a theoretical point of view. Yet, the observed bimodal temperature distribution of WR stars in the SMC cannot be explained by existing single-star or binary evolution models. To better understand the observed temperature distribution of WR stars in the SMC, the role of the evolutionary secondary and its response to mass transfer is studied here in detail. To achieve this, I calculated a small grid of binary evolution models at the SMC metallicity, that follows the evolution of both the primary and secondary stars in detail. The analysis of the new binary evolution models suggests that hot (T ≈ 100 kK), hydrogen-poor WN-type stars are the descendants of “ordinary” primary stars or secondaries that have accreted less than a few percent of their initial mass. In contrast, moderate-temperature (T ≈ 50 kK) WN stars can emerge through two channels: (i) as former accretors that underwent rejuvenation, altering their interior structure and leading to higher surface oxygen abundances (XO = 20×10−5) after mass-transfer, or (ii) as luminous stars experiencing envelope inflation, that exhibit surface oxygen abundances in accordance with the CNO-equilibrium value (XO = 2×10−5). The first observational evidence supporting this hypothesis comes from the WR star SMC AB 4, whose optical spectra can only be explained with a stellar atmosphere model having a surface oxygen abundance one order of magnitude higher than the CNO-equilibrium value. Following the binary evolution models, secondary stars that have accreted mass relatively conservatively and, thus, got rejuvenated will evolve at low metallicity into WN stars with moderate temperatures and should be accompanied by a compact object. This would imply that the WN-type star SMC AB 4 should have a so far unseen compact companion. Future spectroscopic surveys focusing on surface oxygen abundances and multiplicity among SMC WR stars are key to further understanding massive star evolution and the role of binarity in forming WR stars in low-metallicity environments.
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This work is distributed under the Creative Commons CC BY 4.0 Licence.
Paper presented at the 41st Liège International Astrophysical Colloquium on “The eventful life of massive star multiples,” University of Liège (Belgium), 15–19 July 2024.
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Acerca de: Daniel Pauli
email : dpauli@astro.physik.uni-potsdam.de